[PATCH] RDMA/rxe: fix responder UAF on IB_QP_MAX_DEST_RD_ATOMIC modify_qp

[Ibrahim Hashimov <security@auditcode.ai>]

rxe_qp_from_attr()'s IB_QP_MAX_DEST_RD_ATOMIC branch frees and
reallocates qp->resp.resources[] (the rd_atomic resource array used by
the responder to track in-flight RDMA READ/ATOMIC/FLUSH requests)
completely outside of the IB_QP_STATE handling above it. Unlike every
other place that tears this array down -- rxe_qp_reset(), reached only
under IB_QP_STATE, always calls rxe_disable_task(&qp->recv_task) /
rxe_disable_task(&qp->send_task) to drain the responder and requester
tasks before touching per-QP state, then re-enables them -- this branch
runs with the responder task (rxe_receiver(), scheduled as recv_task on
the rxe_wq workqueue) fully live and unlocked. A userspace modify_qp()
that sets only IB_QP_MAX_DEST_RD_ATOMIC (no state change, so
__qp_chk_state()/ib_modify_qp_is_ok() never runs and qp->state_lock is
never taken here) can therefore race the responder in two ways:

 1. free_rd_atomic_resources() calls kfree(qp->resp.resources) and
    alloc_rd_atomic_resources() kzalloc_objs()'s a new array while
    rxe_prepare_res()/find_resource() in rxe_resp.c are concurrently
    walking &qp->resp.resources[i] with no lock held -- a straight
    free-vs-read race on the array itself.

 2. free_rd_atomic_resources() only NULLs qp->resp.resources; it never
    clears qp->resp.res, the raw pointer *into* that array that
    rxe_resp.c caches across a multi-packet RDMA READ/ATOMIC/FLUSH
    reply (set at rxe_resp.c read/atomic/flush-reply sites, cleared
    only on the normal completion paths). If a modify_qp() races a
    resource still referenced by qp->resp.res, the array is freed out
    from under the cached pointer and the next reply packet dereferences
    it -- independent of the kfree/kzalloc_objs() window in (1).

Reproduced with KASAN: a single process driving one RC QP pair in rxe
loopback, one thread pumping large multi-packet IBV_WR_RDMA_READs
against qpB while a second thread hammers
ibv_modify_qp(qpB, IB_QP_MAX_DEST_RD_ATOMIC), reliably (~11s) produces

  BUG: KASAN: slab-use-after-free in rxe_receiver+0x4f78/0x89e0 [rdma_rxe]
  Workqueue: rxe_wq do_work [rdma_rxe]

with the freed kmalloc-1k object being the rd_atomic resource array
freed by the modify_qp() thread while the recv_task kworker reads it.
An identical run modifying only IB_QP_MIN_RNR_TIMER (no resource free)
is clean.

Fix both races the same way rxe_qp_reset() already handles tearing down
this exact array: quiesce the responder task around the free/realloc by
calling rxe_disable_task(&qp->recv_task) before free_rd_atomic_resources()
and rxe_enable_task(&qp->recv_task) after alloc_rd_atomic_resources(),
so rxe_receiver() cannot observe the array mid-free/mid-realloc. And
close the still-open window for (2) at the source: have
free_rd_atomic_resources() clear qp->resp.res along with
qp->resp.resources, exactly like the existing completion paths in
rxe_resp.c (check_rkey()/duplicate_request()/RESPST_CLEANUP) already do
when a resource's lifetime ends, so a drained-and-resumed responder
restarts at RESPST_CHK_PSN against the fresh array instead of replaying
a stale reference into the old one.

Only qp->recv_task is drained: qp->resp.resources / qp->resp.res are
touched exclusively by the responder (rxe_resp.c); the requester
(send_task / rxe_sender()) never reads them, so there is no need to
widen this beyond what rxe_qp_reset() would drain for the equivalent
state.

Verified on the same v6.19 KASAN stand: with this fix applied, the
identical differential reproducer drives sustained MAX_DEST_RD_ATOMIC
storms against qpB well past the ~11s pre-fix time-to-first-splat with
zero KASAN reports, versus reliably tripping the slab-use-after-free in
rxe_receiver() described above before the fix.

Fixes: 8700e3e7c485 ("Soft RoCE driver")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
 drivers/infiniband/sw/rxe/rxe_qp.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/sw/rxe/rxe_qp.c b/drivers/infiniband/sw/rxe/rxe_qp.c
index f3dff1aea96a..646957707765 100644
--- a/drivers/infiniband/sw/rxe/rxe_qp.c
+++ b/drivers/infiniband/sw/rxe/rxe_qp.c
@@ -172,6 +172,7 @@ static void free_rd_atomic_resources(struct rxe_qp *qp)
 		}
 		kfree(qp->resp.resources);
 		qp->resp.resources = NULL;
+		qp->resp.res = NULL;
 	}
 }
 
@@ -709,9 +710,15 @@ int rxe_qp_from_attr(struct rxe_qp *qp, struct ib_qp_attr *attr, int mask,
 
 		qp->attr.max_dest_rd_atomic = max_dest_rd_atomic;
 
+		/*
+		 * Not gated by IB_QP_STATE above: quiesce the responder task
+		 * the same way rxe_qp_reset() does before touching this
+		 * array, so rxe_receiver() can't race the free/realloc.
+		 */
+		rxe_disable_task(&qp->recv_task);
 		free_rd_atomic_resources(qp);
-
 		err = alloc_rd_atomic_resources(qp, max_dest_rd_atomic);
+		rxe_enable_task(&qp->recv_task);
 		if (err)
 			return err;
 	}
-- 
2.50.1 (Apple Git-155)